Contouring is a known video/image artifact that is particularly noticeable in smooth luminance or chrominance regions, particularly in dark regions with smooth chrominance. Contouring is also noticeable where a large amount of content is compressed using block transforms (i.e., 8×8 discrete cosine transforms and 4×4 integer discrete cosine transforms). For areas with gradual chroma gradient, digital cinema people, and to a lesser extent, high definition DVD people are very concerned with the ability of existing 8-bit video to avoid contouring. Some conventional solutions to contouring include dithering and noise addition. Other conventional solutions involve increasing a bit depth of the video equipment from end to end (i.e., from an 8-bit depth to a 10-bit depth).
Recently, consumer liquid crystal display (LCD) manufacturers, such as Samsung and Sharp, have been developing 10-bit consumer LCD displays. The 10-bit displays are likely to become an increasingly common device-output and display format. Therefore, an incentive exists to decompress available 8-bit compressed video content with as much accuracy as possible.
Existing solutions for decoding 8-bit transform compressed video/images are not as accurate as possible. Just before a last stage of an inverse transform, conventional solutions discard (i.e., round away) several bits of information that are available about how the compressed video bitstream has approximated an original video signal. Therefore, solutions that attempt to produce 10-bit video from 8-bit decoded video (the output of a standard transform decoder) are at a disadvantage to a solution that retains the information available in the normally discarded bits. The discarded bits represent a best known approximation of the additional bits that are available from the compressed bitstream but thrown away.